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quality. The only universality in this attribute more than in other attributes of matter is found in the fact, that all matter must exist in space, and hence under the one form of extension. Space, extension, is a necessary condition of matter. Without it, those qualities, properly so called, which constitute matter, cannot have a being. It is involved in their manifestations, that they occupy some portion of space, and this primary quality, so called, is only this essential condition for the existence of matter. We might as well say that duration is a quality of matter, as to say that extension is such a quality; since no form of matter can exist without occupying some period of time, more or less.

Nor is the second criterion any more satisfactory in its application. If there is any one direction in which the mind acts with a sense of establishing and defining its own data, it is this of extension. Odor, taste, color, are what they are, directly, through the nature of the outside cause; but the form of a body is arrived at through meagre grounds of judgment unfolded by the enlargement and corrections of protracted experience; while the notion under which alone this evolution can proceed, that of space, is furnished entirely by the mind. Perception, instead of being unusually direct and immediate in extension, is more than elsewhere indirect and enlarged by inference.

Solidity, as a primary quality, is open to a like form of criticism. That which must in this discussion be understood by solidity, is very different from the notion which the word ordinarily conveys; it is the impossibility of complete compression, complete displacement. A gas is in this sense as much a solid as a piece of steel, since, when properly confined in a cylinder, it is found to exclude the piston as certainly as the most solid substance. Compression cannot proceed to all lengths. Resistance accompanies pressure,